1. The germ-plasm has had unbroken continuity from the beginning of life. Owing to its impressionable nature, it has an inherited organization of great complexity. 2. Heredity is accounted for on the principle that the offspring is composed of some of the same stuff as its parents. The body-cells are not inherited, _i.e._, 3. There is no inheritance of acquired characters. 4. Variations arise from the union of the germinal elements, giving rise to varied combinations and permutations of the qualities of the germ-plasm. The purpose of amphimixis is to give rise to variations. The direct influence of environment has produced variations in unicellular organisms. 5. Weismann adopts and extends the principle of natural selection. Germinal selection is exhibited in the germ-plasm. IV. De Vries's Theory of Mutations. 1. The formation of species is due not to gradual changes, but to sudden mutations. 2. Natural selection presides over and improves variations arising from mutation. Among the other theories of evolution that of Eimer is the most notable. He maintains that variations in organisms take place not fortuitously or accidentally, but follow a perfectly determinate direction. This definitely directed evolution is called orthogenesis. He insists that there is continuous inheritance of acquired characters, and he is radically opposed to the belief that natural selection plays an important part in evolution. The title of his pamphlet published in 1898, _On Orthogenesis and the Impotence of Natural Selection in Species-Formation_, gives an indication of his position in reference to natural selection. A consideration of Eimer's argument would be beyond the purpose of this book. The cause for the general confusion in the popular mind regarding any distinction between organic evolution and Darwinism is not far to seek. As has been shown, Lamarck launched the doctrine of organic evolution, but his views did not even get a public hearing. Then, after a period of temporary disappearance, the doctrine of evolution emerged again in 1859. And this time the discussion of the general theory centered around Darwin's hypothesis of natural selection. It is quite natural, therefore, that people should think that Darwinism and organic evolution are synonymous terms. The distinction between the general theory and any particular explanation of it has, I trust, been made sufficiently clear in the preceding pages. CHAPTER XIX THE RISE OF EVOLUTIONARY THOUGHT